Heart disease such as myocardial infarction and/or heart failure can cause adverse ventricular remodeling and an imbalance in autonomic tone favoring sympathetic activity over parasympathetic tone. During heart disease, the compromised ventricles may be less than capable of maintaining normal cardiac output. As a result, the body compensates for the reduced cardiac output by increasing sympathetic tone and suppressing parasympathetic activity, resulting in increased heart rate, myocardial contractility and blood volume. This mechanism is acutely beneficial, but has a long-term deleterious effect.
It has been shown experimentally that intermittent stress such as exercise, dobutamine infusion, myocardial pacing, or external counterpulsation provides beneficial conditioning effects for the heart and body. Intermittent stress (e.g. exercise) improved the imbalance in the autonomic tone, as the autonomic tone trended from a predominantly sympathetic tendency toward a desired autonomic balance between the sympathetic and parasympathetic systems. For example, intensive exercise training in patients with reduced ventricular function has been shown to result in a significant improvement in exercise capacity (increased O2 uptake, maximum minute ventilation, CO2 production, exercise time and watts), with no deleterious effects on left ventricular volume, function or wall thickness. A potential mechanism for the benefit may be that these short intervals of stress increase sympathetic tone and cause a reflexive increase in parasympathetic tone after the stress is discontinued.
Intermittent sympathomimetic stimulation in animals with dobutamine produces benefits analogous to those of physical conditioning. In a pilot clinical study, patients with stable moderate severe HF (EF=23%) who received dobutamine therapy (30 min/day, 4 days/week, 3 weeks) experienced the following benefits: increased exercise tolerance; improved heart rate variability; lowered peripheral vascular resistance; and reduced plasma noradrenaline.
It has been proposed to deliver intermittent stress in the form of artificial cardiac pacing as a potential therapy for cardiac disease. A patient may not experience the desired benefit if the pacing delivers too little stress, or may be harmed (similar to over-exercising) if the pacing delivers too much stress.